The present invention relates to a toner image transcription transfer unit in an electrophotographic copying machine.
The present invention relates, more particularly, to a control system for stabilizing a transcription operation without regard to the ambient humidity.
Generally, in an electrophotographic copying machine, a toner image is formed on a photosensitive drum through the use of a uniform charging corona unit, an image exposure system and a developing unit. The thus formed toner image is transcribed or transferred onto a copy paper through the use of a transcription or transfer corona unit. The transcription corona unit functions to apply charges to the copy paper of the same polarity as the electrostatic image formed on the photosensitive drum and of the opposite polarity to the charges carried by the toner, thereby electrostatically attracting the toner attached to the photosensitive drum surface.
Therefore, it is required that sufficient charges are applied to the copy paper for ensuring a stable transcription operation.
The copy paper is normally stored in a copy paper cassette and supplied to the transcription unit through the use of a copy paper supplying system. The copy paper supplying system includes a paper guide disposed near the transcription corona unit. The paper guide is made of metal and the copy paper is transferred along the paper guide in a fashion that the copy paper contacts the paper guide.
In a high humidity condition, the resistance value of the copy paper is reduced so that the charges supplied from the transcription corona unit flow through the paper guide to the body of the electrophotographic copying machine. This leakage of the charges will deteriorate the transcribed toner image.
The present inventors have discovered that the above-mentioned leakage of charges is precluded when the paper guide is electrically isolated from the body of the electrophotographic copying machine. Thus, a clean copy is obtained even at the high humidity condition. However, if the paper guide is insulated in the normal humidity condition or in the low humidity condition, the charges are accumulated on the paper guide. The thus accumulated charges will cause a spark between the paper guide and the body of the electrophotographic copying machine. The spark creates noises which prevent the accurate operation of the electronically controlled copying machine.
Accordingly, an object of the present invention is to provide a toner image transcription system which ensures a clean copy without regard to the ambient humidity.
Another object of the present invention is to provide a switching system for selectively grounding a paper guide which is disposed near a transcription corona unit.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a switching system is connected to a paper guide which is disposed near the transcription corona unit. In the high humidity condition, the switching system is switched on to electrically isolate the paper guide from the body of the copying machine. In the normal humidity condition or the low humidity condition, the switching system is switched off to electrically connect the paper guide to the body of the copying machine.